Explain the design procedure of Offline Switch mode Power Supplies.
Power supply specifications |
| The first step in designing a switching mode power supply is to determine
the power supply specifications. Satisfy the client's requirements, investigate safety and EMC standards, and create a power supply specification document. |
Selection of the main device |
| Select an appropriate main device based on the switching power supply specifications.
We recommend devices with the following built-in features: -Low standby power consumption when no load is present -Slow start at startup -Built-in output overvoltage protection -Built-in device overheat protection -Oscillator with frequency jitter |
Transformer design |
| Calculate the appropriate transformer based on the switching power supply
specifications and main device. Create a transformer drawing from the transformer calculation results. The transformer structure changes depending on the purpose, such as prioritizing efficiency or low noise. When designing a transformer, increasing the magnetic flux density increases the risk of magnetic saturation, while decreasing the magnetic flux density makes the transformer larger and increases costs. Magnetic saturation of the transformer will destroy the main device. |
Schematics and bill of materials |
| Create a circuit diagram and bill of materials by referring to the data
sheet for the main device, which contains the basic circuit and default circuit constants. The components used in a switching power supply vary depending on the required electrical specifications, safety standards, and EMC standards. The derating margin for components is the designer's know-how. |
PCB design |
| The data sheet for the main device may include an example PCB design, which can be used as a reference when designing the PCB. The PCB design is an important factor that affects EMC noise and stability performance. PCB design involves appropriate component placement and pattern wiring. |
Switching waveforms |
| After confirming that the switching power supply board is working properly, measure the switching voltage and current waveforms of the main device. Here, check for magnetic saturation or unstable operation. |
Regulation and efficiency |
| Check if the input/output characteristics and efficiency are as specified. Some main devices have an output overvoltage protection function that utilizes the auxiliary winding voltage, and stable regulation of the auxiliary winding is also required. Also check the power consumption under no load. If efficiency is significantly lower than expected, there may be a problem with the design. |
EMC noise |
| EMC noise is divided into conductive noise and radiated noise, and the measurement method and specified values are determined by noise standards. The method of suppressing noise differs depending on the frequency band. |
Output noise |
| Check if the output ripple and spike noise meet the specifications. |
Protective function |
| Check the operation of overload protection and output overvoltage protection. |
Dynamic characteristics |
| Check the rise waveform and control characteristics of the output voltage. |
Temperature rise and life |
| Calculate the temperature rise of key components and estimated lifetime of electrolytic capacitors. |
De-Rating |
| Check margin for maximum ratings for all components. |
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| ※Reliability tests and safety tests are omitted because they are not the job of design engineers. |
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